1. Field of the Invention
The present invention generally relates to an image sensing device typified by a photosensing device (photodetecting device) in one-dimensional and two-dimensional image reading devices utilized in facsimile machines, digital copying machines, scanners, and so on, and to a production process thereof and, more particularly, the invention relates to an image sensing device well suited for the application for wavelength-converting a radiation such as X-ray or xcex3-ray to a photosensitive wavelength region such as visible light by a fluorescent screen and reading the converted light, and to a production process thereof.
2. Related Background Art
Heretofore, as a reading system used for input of an image information in the facsimile machines, digital copying machines, radiation sensing apparatuses or the like, there has been used an optical system using an image reducing optical system and a CCD sensor. However, in recent years, the development of photoelectric converting semiconductor material typified by amorphous silicon (hereinafter abbreviated as a-Si film) has forwarded the development of a contact type sensor in which a photoelectric conversion element is formed on a large-area substrate and in which an image information is read with an optical system having 1:1 magnification to an information source, which contact type sensor is being put to practical use.
Since the a-Si film can be used not only as photoelectric converting material but also as semiconductor material for a switching TFT, it possesses an advantage that semiconductor layers of the photoelectric conversion element and semiconductor layers of the switching TFT can simultaneously be formed.
As a typical example of a photosensor using this a-Si film, there is included a pin type photosensor such as illustrated in a schematic sectional view of FIG. 1. Reference numeral 101 designates a glass substrate, 102 a lower electrode, 103 a p-type semiconductor layer (hereinafter abbreviated as a p-layer), 104 an intrinsic semiconductor layer (hereinafter abbreviated as an i-layer), 105 an n-type semiconductor layer (hereinafter abbreviated as an n-layer), 106 a transparent electrode, and 107 incident light. The photosensor of FIG. 1 has the structure in which the layers are stacked in the above-mentioned order on the glass substrate 101.
This photosensor can be operated by use of a circuit configuration such as illustrated in FIG. 2, in which reference numeral 110 denotes a pin type sensor, 111 a power source, and 112 a detector such as a current amplifier. In the photosensor 110, the side C represents the side of the transparent electrode 106 while the side A the side of the lower electrode 102. The voltage of the power source 111 is set such that a positive voltage is applied to the side C relative to the side A of the photosensor 110.
The basic operation of this pin type photosensor will be outlined referring to FIG. 1 and FIG. 2. When the light 107 is incident in the direction indicated by the arrow as illustrated in FIG. 1, the incident light undergoes photoelectric conversion in the i-layer 104 to generate electrons and holes. Since the power source 111 applies an electric field to the i-layer 104, the electrons move toward the side C, i.e., through the n-layer 105 into the transparent electrode 106, while the holes move toward the side A, i.e., to the lower electrode 102. This means that a photocurrent flows in the photosensor 110.
Further, when there is no incidence of light 107, neither electrons nor holes are generated in the i-layer 104. For the holes in the transparent electrode 106, the n-layer 105 works as a hole injection inhibiting layer; for the electrons in the lower electrode 102, the p-layer 103 works as an electron injection inhibiting layer. As a consequence, neither electrons nor holes can move, so that no electric current flows. As described, the electric current to flow in the circuit varies depending upon presence or absence of incidence of light. The operation as a photosensor is achieved by detecting the current by the detector 112 of FIG. 2.
However, it is not easy for the above pin type photosensor to realize a photosensing device of a high S/N ratio and a low cost for the following reasons. The first reason is that the pin type photosensor has to include the injection inhibiting layers of the p-layer and n-layer. This is because in the pin type photosensor of FIG. 1 the n-layer 105 as the injection inhibiting layer needs to have characteristics to guide electrons to the transparent electrode 106 and to inhibit holes from entering the i-layer 104. If either one of the characteristics is missed, the photocurrent will decrease or the current without incidence of light (hereinafter referred to as dark current) will appear and increase, which will cause lowering in the S/N ratio. In order to improve the characteristics, it is normally necessary to optimize the film quality of the i-layer 104 and the n-layer 105, i.e., to optimize film-forming conditions, particularly, various conditions including thermal treatment conditions after production of the layers.
On the other hand, the p-layer 103, although having the reverse relation between electrons and holes, also needs to have characteristics to guide holes to the lower electrode 102 and to inhibit electrons from entering the i-layer 104 and, just as in the case of the n-layer 105 described above, it is also necessary to optimize the respective conditions for the i-layer 104 and p-layer 103. In other words, in general, there is a difference in conditions for the optimization of the n-layer and for the optimization of the p-layer and it is difficult to satisfy the conditions of the two optimizations simultaneously. Namely, there is a possibility that the necessity for the injection inhibiting layers of p-layer and n-layer in the same photosensor may be an obstacle to formation of a photosensor with a high S/N ratio.
The second reason will be described referring to FIG. 3. FIG. 3 schematically shows a switching TFT. This TFT is utilized as a part of a control section in formation of a photosensing device. In the figure, reference numeral 101 designates a glass substrate, 102 a lower electrode, 107 an insulating film, 104 an i-layer, 105 an n-layer (or n+-layer), and 160 an upper electrode.
This switching TFT is produced in the following sequence. The lower electrode 102 functioning as a gate electrode G, the gate insulating film 107, the i-layer 104, the n-layer 105, and the upper electrode 160 functioning as source-drain electrodes (hereinafter abbreviated as S-D) are successively formed on the glass substrate 101, the upper electrode 160 is etched to form the source-drain electrodes, and thereafter the n-layer 105 is partly removed to form a channel portion 170.
Since the switching TFT has such a property as to be sensitive to the interface state between the gate insulating film 107 and the i-layer 104, it is normally preferable that the production process described above be carried out in the form of continuous film formation without breaking vacuum.
When the pin type photosensor described above is formed on the same substrate as this switching TFT is, the aforementioned layer structure would cause increase of production cost and lowering in the characteristics. The reason is the difference in the layer structure sequence between them; the photosensor illustrated in FIG. 1 has the structure of the electrode, p-layer, i-layer, n-layer, and electrode in this order from the substrate side, whereas the switching TFT has the structure of the electrode, insulating layer, i-layer, n-layer, and electrode in this order from the substrate side.
This means that the photosensor and the switching TFT cannot be produced simultaneously by the same process. In other words, they are produced by complex processes in which many film-formation and photolithography steps and so on are repeated to form necessary layers in necessary areas, which lowers the yield but increases the cost.
For example, if the i-layer and n-layer are used in common to the pin type photosensor and the switching TFT in order to simplify the production process, it will be possible at least to continuously deposit the gate insulating layer and the p-layer, to remove the p-layer in the switching TFT part, and thereafter to continuously form the i-layer and the n-layer. However, this will result in contaminating the important interface between the gate insulating film and the i-layer of the switching TFT and the interface between the p-layer and the i-layer of the pin type photosensor to cause degradation of the characteristics and lowering in the S/N ratio.
It is also hard to produce a capacitive element (hereinafter referred to as a capacitor), which is necessary for obtaining an integral of charge or current generated by the pin photosensor, with such good characteristics as to demonstrate little leak and in the same structure as that of the photosensor. That is, it is absolutely necessary for the capacitor to have a layer for inhibiting movement of electrons and holes as an intermediate layer between two electrodes, for accumulating charge between the two electrodes. However, since the layer structure of the pin photosensor uses only the semiconductor layers between the electrodes, it is not formed into a good capacitor with little leak.
As described above, under such conditions that matching is not achieved in terms of the process or in terms of the characteristics in producing the switching TFT and the capacitor which are important elements for constituting the photosensing device, the steps inevitably become complicated and the yield is lowered. The points stated above might pose significant issues in some cases, particularly, where the photosensing device having a lot of photosensors arranged one-dimensionally or two-dimensionally and having a function of successively detecting photosignals therefrom is implemented as a high-performance and multi-functional device at a low cost.
The present invention has been accomplished under the above circumstances and an object of the invention is to provide a photosensing device and a production process thereof that permit formation of the photoelectric conversion element and switching TFT with a high S/N ratio and with stable characteristics by one and the same process.
Another object of the present invention is to implement an inexpensive photosensing device with a high S/N ratio by producing a photoelectric conversion element comprising a first electrode layer, an insulating layer, a photoelectric conversion semiconductor layer, a carrier injection inhibiting layer for inhibiting a carrier from being injected into the semiconductor layer, and a second electrode layer, as an MIS (Metal Insulator Semiconductor) type photoelectric conversion element, and a TFT comprising a first electrode layer, an insulating layer, a semiconductor layer, an ohmic contact layer to the semiconductor layer, and a second electrode layer, as a switching TFT (Thin Film Transistor), on an insulating substrate by the same and simplified production process.
According to one aspect of the present invention, there is provided a process of producing a photosensing device having at least a photoelectric conversion element and a switching TFT, comprising the steps of:
a) forming a first electrode layer by use of a first mask;
b) successively stacking an insulating layer, a semiconductor layer, and an n+-type semiconductor layer on the electrode layer;
c) forming a contact hole for connection of a second electrode layer by use of a second mask;
d) patterning the second electrode layer to serve at least as source-drain electrodes of the switching TFT, by use of a third mask;
e) removing the n+-type semiconductor layer exposed between portions of the second electrode layer;
f) patterning the second electrode layer in an area except for the source-drain electrodes of the switching TFT, by use of a fourth mask; and
g) effecting isolation between elements by use of a fifth mask.
According to another aspect of the present invention, there is provided a photosensing device comprising a pixel comprising a photoelectric conversion element and a switching TFT arranged corresponding to the photoelectric conversion element on a substrate, and a wire associated with the pixel, wherein at least a part of the wire is of a layered structure through a contact hole.
According to still another aspect of the present invention, there is provided a process of producing a photosensing device having a photoelectric conversion element and a TFT, comprising carrying out patterning of source-drain electrodes of the TFT part and patterning of an upper electrode of the photoelectric conversion element separately in different steps.